1. Field of the Invention
The present invention relates to hot runner injection molding systems, and more particularly to a thermal expansion compensation support for a manifold used in a hot runner injection molding system.
2. Description of the Related Art
In hot runner injection molding systems a heated manifold is provided to convey molten plastic molding material from a source, such as a plastication barrel, to a plurality of injection nozzles. The nozzles are in fluid communication with respective mold cavities that define the shapes of parts to be molded. The manifold is heated to maintain the molten plastic material at a desired elevated temperature so that the material does not cool excessively as it flows from the plastication barrel to the mold cavities. The heat is typically provided by electrical heating elements within the manifold, or by circulating heated fluids through passageways within the manifold. The added heat maintains the molding material in a fluid state so it can readily be conveyed and completely fill the respective mold cavities to provide fully formed molded parts. Although the manifold is heated, the adjacent structural elements of the injection molding machine, which generally include a clamp plate and an injection nozzle retainer plate, are not heated and may actually be cooled by adjacent mold elements.
The hot runner manifold is generally spaced from the adjacent structural elements of the machine by spacers or supports, which are often disc-shaped or annular metallic members that serve to support the hot runner manifold within the mold assembly and space the manifold from the adjacent mold elements. The molding material is conveyed to the hot runner manifold, and then conveyed through the manifold to the respective injection nozzles. When starting such an injection molding machine from a xe2x80x9ccoldxe2x80x9d (start-up) condition, the hot runner manifold is initially spaced from the respective adjacent machine structural elements at a predetermined distance. This spacing distance diminishes when the manifold expands as its temperature increases during the course of the operation of the injection molding machine. However, the adjacent structural elements, which are not directly heated, are at a lower temperature and therefore expand to a lesser degree.
As the molten molding material is conveyed from the plastication barrel to the hot runner manifold and from the manifold to the respective mold cavities, it passes through flow passageways that must remain aligned with each other to prevent leakage of the fluent molding material. Thus, it is essential that the molding material flow passageways within the respective adjoining elements of the machine be properly aligned throughout the machine warm-up process and subsequent operation, even though the parts expand at different rates and may ultimately have different operating temperatures.
In the past, various structural arrangements have been proposed in an effort to ensure that the respective parts of a hot runner injection molding machine are properly aligned and are arranged in leak-tight relationship. For example, in U.S. Pat. No. 4,588,367, entitled xe2x80x9cHot Runner Manifold For Injection Molding Machinexe2x80x9d, which issued on May 13, 1986, to Schad, the injection nozzle is retained in sealing engagement with a hot runner manifold block by means of a pair of Belleville washers. These springs are positioned to maintain engagement between the injection nozzles and the manifold block from initial start-up, through warm-up, to normal operating temperature. However, if the springs were to fail either before or during the time the machine is at normal operating temperature, the failure of the springs would allow the flow passageway between the injection nozzle and the manifold block to open. The parts would separate as a result of removing the spring force, thereby allowing the molten molding material to leak from the open flow passageway into the space between the manifold block and the mold plate, possibly interfering with the molding process by not supplying sufficient material to form the part properly.
Another arrangement for maintaining contact between a manifold block and a nozzle to avoid molding material leakage involves the use of a somewhat flexible spacer. Such an arrangement is disclosed in U.S. Pat. No. 5,125,827, entitled xe2x80x9cInjection Molding Apparatus Having An Insulative And Resilient Spacer Memberxe2x80x9d, which issued on Jun. 30, 1992, to Gellert. That patent discloses the use of an annular metallic spacer that is positioned between a hot runner manifold and the clamp plate that contacts the plastication barrel. The spacer is defined by a plurality of peripherally interconnected, V-shaped concentric rings that allow the spacer to deflect during the expansion of the manifold block relative to the clamp plate during warm-up to maintain the parts that define the material flow passageway in contact with each other. However, the structure of the spacer and the elasticity of the metallic material from which it is formed limits the degree of deflection that the spacer can undergo, and therefore full sealing contact of the injection nozzle and the manifold block throughout the range from cold start-up to full operating temperature cannot be achieved.
It is an object of the present invention to overcome the deficiencies of the spacer configurations in the prior art arrangements. It is another object of the present invention to provide a thermal expansion compensation support that is effective to maintain the parts that define the material flow passageway in continuous contact throughout the range of operation of a hot runner injection molding machine, from cold start-up through normal operating temperature, without allowing leakage of molding material from between the adjoining mold elements within which the molten material flow channel is provided.
Briefly stated, in accordance with one aspect of the present invention, a thermal expansion compensation support is provided in a hot runner mold assembly. The support includes a housing having a recess that defines an opening in the housing. A spring is positioned within the housing recess and extends outwardly of the housing opening. A cover overlies the opening and is in surface contact with the spring. A connector extends between the housing and the cover for holding the cover against the spring, thereby compressing the spring to apply a xe2x80x9cpreloadxe2x80x9d to the support, while leaving a defined gap between the housing opening and the cover.
In accordance with another aspect of the present invention, a method is provided for liquid-tight interconnections between the several members of the injection mold construction in which the molding material flow passageway is contained. The method includes holding the members together at a first holding force level for a first portion of the predetermined temperature range, and holding the members together at a second force level for a second portion of the predetermined temperature range.